Earthworms buffer the impacts of nitrogen enrichment on energy dynamics of soil micro-food webs

Anthropogenic nitrogen (N) enrichment is an important driver of global soil biodiversity loss, particularly for large-bodied consumers at the higher trophic levels of food webs. This driver is predicted to vastly impact the energy dynamics in soil food webs, which underpin ecosystem functioning and services. Yet, we still lack empirical evidence about how N-enrichment and the loss of large consumers (e.g., earthworms) might affect the energetic structure of soil food webs and associated ecosystem functions. Here, based on a 4-year field manipulation experiment, we explore the interactive effects of increasing N inputs and earthworms on the energy dynamics and trophic functions (i.e., herbivory, decomposition and predation) of soil micro-food webs. Our results revealed that after earthworm removal, total and average energy flux of soil micro-food webs decreased linearly as N input increased, largely explained by functional diversity. Specifically, decomposition, as indicated by energy flux through decomposers, initially decreased and then increased with increasing N inputs, while herbivory and predation decreased linearly. However, earthworm activities mitigated such negative effects of N enrichment on energy dynamics, maintaining total and average energy flux largely unchanged across the N gradient. Along with functional diversity, we also found that earthworm-induced changes in taxonomic diversity was positively correlated with total and average energy flux, possibly attributed to facilitating species interactions and thus fostering energy transfers. These findings emphasize the importance of protecting large consumers as biotic buffers to counteract biodiversity loss and maintain trophic functions under future N enrichment.